This invention relates in general to maintaining the purity of a high purity material in a pressurized system and dispensing such material. This invention relates in particular to processes, apparatuses, and valves for dispensing a high purity material from a pressurized vessel by depositing the high purity material on a noncontaminating surface and then transferring it only when a gastight valve is opened to dispense the solid material.
Various types of standard valves have been used in the past to dispense gaseous, solid, and liquid materials. However, applicants have found that standard valves are unacceptable for dispensing relatively hard solids that may cause valve wear. In preserving the high purity of a granular or similar solid product, valve wear also has the disadvantage of contaminating the solid product with the metal, elastomers, or other material from which the valve is constructed. That is, not only do the valves wear out prematurely and lose their gas seal but for high purity applications, the material that wears the valves becomes contaminated with impurities from the valve body. For example, a mechanically operated or gas operated elastomeric pinch valve has been used to shut the opening of the valve, trapping between the elastomatic walls of the valve particles of granular or similar solid product but making a gastight seal. The continued opening and closing of the elastomatic valve continues to wear on the walls of the valve so as to make the valve lose its gas seal. Similarly, ball valves and butterfly valves wear and lose their gas seal. In the process of losing the gas seal, such valves concurrently contaminate a high purity granular or similar material with carbon, phosphorus, boron, nickel, or other elements present in the valve walls. Metals are particularly troublesome, especially for dispensing semiconductor materials.
When a high purity granular material such as silicon granules, gallium arsenide particles, or other III-V semiconductor materials, for example, are contained in a pressurized vessel, they will necessarily become somewhat contaminated by use of only a standard, gastight valve as described above. While this contamination may seem very minor to the casual observer, the contamination is very significant when the purity of the granules or similar material is to be maintained in the parts per million, parts per billion, or even parts per trillion range.
In certain pressurized vessels, the granules or other forms of solid are at relatively high temperature which can accelerate the process of wearing out the valve and increase the rate of contamination of the high purity material from the walls or other surfaces of the valve.